Blue Genes: Cell Division and Down Syndrome Notes

Gametes and Cell Division

• Gametes are human reproductive cells: sperm (males, produced in testicles) and ova (females, produced in ovaries).

• Mature gametes are haploid ($23$ chromosomes). Fusion during fertilization forms a zygote with $46$ chromosomes.

• Mitosis: Division of somatic cells for growth, repair, and maintenance. Results in $2$ genetically identical diploid daughter cells.

• Meiosis: $2$-step division producing $4$ genetically unique haploid gametes. Involves genetic recombination in Meiosis I.

• Cell cycle phases follow the order: Interphase, Prophase, Metaphase, Anaphase, Telophase, and Cytokinesis ($\text{PMAT}$).

Comparison of Mitosis and Meiosis

• Mitosis: $1$ division, $2$ diploid daughter cells, occurs in somatic cells, sister chromatids separate in Anaphase.

• Meiosis: $2$ divisions, $4$ haploid daughter cells, occurs in sex cells, sister chromatids separate in Anaphase II.

Causes and Types of Down Syndrome

• Trisomy 21 (Nondisjunction): ~$95\%$ of cases. A pair of $21^{st}$ chromosomes fails to separate; ~$90\%$ of events are maternal and not hereditary.

• Mosaicism: ~$1-$2\%$of cases. Some cells have$46$chromosomes while others have$47. Occurs after fertilization.\n- **Robertsonian Translocation:** ~3-$4\%$ of cases. The long arm of chromosome $21$ attaches to another chromosome (usually $14$). This is the only form that can be inherited from a carrier parent.

• Recurrence Risk: Significantly higher if a parent is a translocation carrier (~$10-$15\%$if maternal; ~$3\% if paternal).\n\n# Karyotyping and Nomenclature\n\n- A karyotype describes chromosome number and structure.\n- Human standard: 46,XX$(female) or$46,XY$(male). Abnormal example:$47,XY,+21 (male with Down syndrome).\n- Chromosome anatomy: p$arm (short),$q arm (long), centromere (joining point), and telomere.\n- Abbreviations: del$(deletion),$dup$(duplication),$t$(translocation),$inv (inversion).\n- **Banding:** G-banding$stains chromosomes with light and dark bands to localize abnormalities (e.g.,$12p3$refers to chromosome$12$, short arm, band$3).\n\n# Screening and Diagnosis\n\n- **Screening (Probability):** \n - Combined First Trimester Screening (\text{CFTS}$): Uses$PAPP-A$, free$\beta-hCG, and nuchal translucency ultrasound.\n - Non-Invasive Prenatal Testing (\text{NIPT}$): Analyzes cell-free fetal DNA from maternal blood ($~10 ext{ weeks} gestation).\n- **Diagnostic (Definitive):** \n - Chorionic Villus Sampling (\text{CVS}$):$11-$12 \text{ weeks}$; ~$1/100$ miscarriage risk.

  • Amniocentesis: $15-$18 ext{ weeks}$; ~$1/200 miscarriage risk.\n\n# Physical and Mental Effects\n\n- **Physical Signs:** Low muscle tone, flattened facial profile, Brushfield spots (iris), Simian crease (palm), and a deep groove between the first and second toes.\n- **Health Risks:** Congenital heart defects (40-$50\%$), vision/hearing loss, and hypothyroidism.

• Mental Impact: Mild to moderate intellectual disability; delays in speech and short-term memory; strong capacity for social and empathetic connection; anxiety is the most prevalent mental health issue.

• Life expectancy in Australia has increased from $15 \text{ years}$ in the $1950s$ to $60 \text{ years}$ today.

Questions & Discussion

• Sharves: What is the difference between balanced and unbalanced chromosomes? Discuss the causes of Down syndrome and how these affect the likelihood of recurrence within families?

  • Response: An unbalanced arrangement involves a gain or loss of genetic material (deletion/duplication), resulting in health issues. A balanced arrangement (translocation) has no loss of material in the parent but increases recurrence risk in offspring. Recurrence is highest in translocation cases ($3-$15\% depending on the carrier parent).\n\n- **Maria:** Are chromosomal and genetic conditions the same? Is Down syndrome a chromosomal or genetic condition?\n - **Response:** All chromosomal conditions are genetic, but not all genetic conditions are chromosomal. Down syndrome is both; while primarily characterized by chromosomal Trisomy 21$$, it is fundamentally a genetic disorder.